The present invention relates to an emission source having a carbon nanotube, and to an electron microscope and an electron beam drawing device using this emission source.
The following four conditions are required to further increase the resolution and brightness of the electron microscope. (1) The size of the emission source needs to be reduced. (2) The brightness of the emission source needs to be increased. (3) The electron energy width from the emission source needs to be reduced so as to reduce the influence of chromatic aberration. (4) An emitted electron beam needs to be stabilized.
There are various unknown types of electron emission source, including a thermal type made of LaB6, a Schottky type made of ZrO/W, and a field emission type made of tungsten in which the tip of a needle is sharpened by electric field polishing. From the point of view of high resolution and high brightness, the emission source of the field emission type is excellent, but it has the following defects. (1) Since no field emission is generated unless a super high vacuum state of 10−8 Pa or more is set, an exhaust system of large-size is required, so that it is difficult to make the device compact, and the cost is increased. (2) Since no field emission is generated unless the drawing-out voltage is set to high value, such as several kV, an organic material and an organism relating sample, which hold promise for rapid growth in the future, are greatly damaged, and so a sufficiently high accuracy observation can not be made. (3) The emission electric current is greatly changed over time by an emitter tip shape change due to the influence of a residual gas molecule adsorbed and desorbed on the emitter surface and due to the impact of a residual gas ion thereon.
In a length measuring SEM (CDSEM) of the type used in a semiconductor manufacturing process, etc., a Schottky type emission source is used at present. However, a higher resolution under a low acceleration voltage has become an important objective to prevent electric charging on the observation sample and to reduce damage to the sample.
Further, in an electron beam drawing device for irradiating an electron beam to a sample substrate that is coated with a resist that is sensitive to the electron beam for forming various kinds of circuit patterns, an emission source for obtaining a micro probe diameter is required, since the various kinds of circuit patterns are highly defined. Thermal electron emission type sources constructed of tungsten and LaB6 are conventionally used However, these emission sources have certain advantages in that the beam electric current can be set to be large, but astigmatism caused by the size of an absolute emitter tip radius is large, so that no drawing of 20 nm or less can be performed. Therefore, the field emission type source has been used recently. However, a new problem exists in that the beam electric current is unstable due to the smallness of the beam electric current and the above-mentioned cause. Accordingly, the exposure amount of the electron beam, i.e., the exposure time, must be increased to reliably perform the drawing, so that there is the disadvantage of a reduction in efficiency.
On the other hand, the use of an emission source constructed by arranging many carbon nanotubes in a plane substrate has recently received considerable attention as a new emission source for a display device. This is because the carbon nanotube has the following characteristics. Namely, since the tip diameter of a carbon nanotube is very small, being at a nano level, the field emission can be performed even at low voltage. Further, since the bonding between carbon atoms is very strong in comparison with a metal, the carbon nanotube is strong against the above-mentioned ion impact, the emission electric current has excellent stability and electrons are emitted even in a relatively low vacuum.
Therefore, if a single carbon nanotube, or a bundle-shaped carbon nanotube made up of several carbon nanotubes apparently set to one bundle, is employed as to the emission source of an electron microscope and the electron beam drawing device, the electron emission site will be at a nano level, so that the electron emission angle will be small and the energy width of the emitted electron will be small. Therefore, high resolution and high definition processing can be performed in comparison with the conventional case.
However, there is almost no known example in which the single carbon nanotube, or the bundle-shaped carbon nanotube having plural carbon nanotubes apparently set to one bundle, is employed as the emission source of an electron microscope and the electron beam drawing device. With respect to the field emission characteristics of the single carbon nanotube, for example, there is only a report of M. J. Fransen, Th. L. van Rooy, P. Kruit, Appl. Surface Sci. 146(1999) 312-327, etc.
The carbon nanotube emission source disclosed in the above-referenced report has a structure in which a carbon nanotube 1 is fixed by carbon contamination 2 on the tip side face of a tungsten needle 3 serving as a base material, as shown in FIG. 1. In such a structure, since the contact area of the tungsten needle 3 and the carbon nanotube 1 is very greatly reduced, the following problems cannot be solved when this structure is employed as the emission source of an electron microscope and an electron beam drawing device. (1) No ohmic contact is affected between the carbon nanotube and the tungsten needle, the electric resistance in a joining portion is increased, and the electric field intensity at the carbon nanotube tip is considerably reduced in comparison with the applied voltage so that the field emission threshold voltage is increased. (2) In a state in which an electric current flows to a certain extent, the supply of an electron to an electron emission site is prevented for the above-stated reasons, and the electric current is saturated even when a greater voltage is applied. Accordingly, no large electric current can be obtained. (3) The amount of heat generated in the joining portion (joint) is increased for the above reasons, so that the tungsten needle serving as a base material is dissolved. (4) Since the strength of the joint is small, the joint is easily seperated by a charge of static electricity, an impact, etc. (5) Since the carbon nanotube is attached to the side face of the tungsten needle, it becomes difficult to adjust the electron beam axis after assembly thereof into an electron gun.
A method of coating the tip portion of an electrically conductive needle with catalyst metallic particles and directly growing the carbon nanotube from the catalyst metallic particles by the CVD method, etc. is known. However, there is no example of manufacture of the carbon nanotube having excellent electron emission characteristics, while simultaneously satisfying the requirements of crystallinity, purity and fineness of the grown carbon nanotube. Further, the diameter of the grown carbon nanotube depends on the diameter of the catalyst metallic particle, and it is necessary to arrange one catalyst metallic nano-particle at the electrically conductive needle tip, so that it is very difficult to manufacture. Even when one carbon nanotube can be grown from the catalyst metallic nano-particle, the catalyst metallic particle is moved in a carbon nanotube growing direction together with the growth of the carbon nanotube. Therefore, the catalyst metallic particle is lost in the joint of the electrically conductive needle and the carbon nanotube, so that the problems caused by the above-stated joining defect can not be solved. Further, when the catalyst metallic particle is left, a problem also exists in that plural carbon nanotubes are grown at random from this catalyst metallic particle.